The present invention relates to a system for purging sulfate from a NOx trap associated with oxidation catalyst-forming means and integrated in an exhaust line of a motor vehicle engine.
More particularly, the invention relates to such a system in which the engine is associated with common manifold or “rail” fuel feeder means for feeding the cylinders of the engine with fuel and adapted, by modifying the engine operation control parameters, to cause the engine to switch between operating with a lean mixture and operating with a rich mixture.
It is known that in order to treat polluting emissions in compliance with regulations, and in particular for diesel-engined vehicles, various types of function are needed, in particular an oxidation function for treating carbon monoxide (CO) and hydrocarbons (HCs), a function of reducing nitrogen oxides, and a filter function associated with a particle-combustion function.
One of the means for reducing nitrogen oxides is to use a NOx trap.
The impregnation in the trap then contains storage elements, e.g. barium, on which nitrogen oxides become fixed in the form of nitrates.
When the trap is exposed to sulfur dioxide (SO2) formed from the sulfur contained in the fuel and the lubricating oil of the engine, sulfates are formed, e.g. barium sulfate, and these are compounds that are more stable than nitrates.
Regenerating the NOx trap then serves to convert the nitrogen oxides, but does not eliminate the sulfates. The trap thus becomes progressively saturated in sulfates, thereby having the effect of reducing the catalytic performance of the trap (NOx conversion and also conversion of CO and HC).
It is therefore necessary to purge sulfates regularly from the trap in order to eliminate the sulfates that have become stored therein.
Purging sulfates from a NOx trap can be performed effectively only under precise conditions of temperature and gas composition.
The medium needs to be rich in reducing agents, so the engine must be operating in rich mode and at high temperature, given that sulfates are compounds that are very stable, thermodynamically speaking.
Two problems then arise for obtaining effective sulfate purging.
Firstly, the higher the temperature, the more effective the desorption, but also the faster the trap is caused to age, which leads to reduced catalytic effectiveness.
It is therefore necessary to avoid heating the trap excessively so as to maintain its performance over the lifetime of the vehicle.
Furthermore, sulfates are released essentially in the form of hydrogen sulfide H2S or of SO2, with other compounds such as COS being given off in much smaller quantities.
Sulfates are preferably desorbed in the form of H2S (an evil-smelling gas) when the medium is deficient in oxygen. This is normally the case while the engine is operating with a rich mixture.
However, the formulation of a NOx trap may contain elements with oxygen storage capacity (OSC) that release oxygen when the medium is poor in oxidizing species.
Thus, when the engine switches over from operating in rich mode to poor mode, the OSC releases oxygen.
Unfortunately, the OSC is not an infinite supply of oxygen and, after a certain length of time, it is used up. Thus, while purging sulfate, when the temperature is high enough to be able to release sulfates, they are initially desorbed in the form of SO2, then when there is no longer enough oxygen in the gas (e.g. the OSC reservoir is empty), they are desorbed in the form of H2S.